The ISO connectionless transport standards

The term Open Systems Interconnection (OSI) has increased in familiarity to those involved in distributed processing and data communications to such an extent that an understanding of the architecture it describes and the principles behind its development have quite nearly become commonplace. OSI consists of two closely related pieces: the OSI Basic Reference Model, ISO 7498, which provides a coordinated framework for the development of protocols to support distributed processing environments; and the set of related protocols which achieve that objective. The Reference Model describes an architecture wherein functions which provide communications are separated into a hierarchy of seven layers. Each layer provides services to the next higher layer, utilizing the services provided by subordinate layers in order to achieve that end. At each layer, protocols are employed to convey information between peer layer entities in communicating systems. When in compliance with the suite of standard protocols prescribed for OSI, systems are considered to be "open". Initially, ISO 7498 defined only the requirements of connection mode services within the Reference Model. However, in recognition of the importance which connectionless-mode applications play in the data communications environment, an addendum to ISO 7498 was produced which defines the requirements for connectionless-mode services in the Reference Model. The purpose of this paper is to present an overview of the development of connectionless-mode service and protocol within the Transport Layer of the OSI Reference Model.